Southern Flounder in Hot Water: How Climate-Dependent Life History Impacts Fisheries Management

Southern Flounder – Texas’s favorite flatfish – are most known for their popularity as a sportfish enjoyed by families, fishermen, and the commercial industry alike. Ranging from South Carolina to Florida and throughout the northern Gulf of Mexico, Southern Flounder are particularly popular in Texas where they are ranked among the top three targeted inshore fish species. The economic importance of this species spans from charter guides to coastal businesses, generating revenue, local jobs, and exciting memories for both new and experienced anglers or gig fishermen.

However, the future of this iconic flatfish is uncertain. Southern Flounder populations have declined throughout their entire range, a trend well documented not only by the Texas Parks and Wildlife Department’s long-running gill net and bag seine surveys, but also by angler harvest rate throughout the years. Adjustments of minimum size, bag limits, and season closures to fend off fishing impacts have had limited success in slowing the decline. This prompts the question: what, if not fishing pressure, is driving the decline? As often seen in fisheries management, there is not one perfect answer; rather, a jumble of compounding factors may be to blame.

The answer likely lies in warming waters caused by climate change. Rising global temperatures, as reported by the National Oceanic and Atmospheric Administration (NOAA), are a prime suspect contributing to Southern Flounder decline. A brief look at Southern Flounder life history is all it takes to understand why water temperature may damage this species’ success. Warming water alters key life history events and may lead to reduced spawning success and larval survival, delayed growth stages, masculinization of sex ratios, and disruptions of migration and spawning patterns. These environmental impacts compound with fishing pressures to compose the challenges fisheries managers face to sustain this vital species.

Warming waters may be inevitable, but many state agencies have sought to aid Southern Flounder by prohibiting harvest during annual season closures. These closures aim to reduce fishing pressure when Southern Flounder are most vulnerable – during their fall spawning migration, or spawning run, when Southern Flounder congregate in tidal inlets (passes) on their yearly journey offshore. Reducing fishing pressure during this time protects the spawning stock from harvest and allows Southern Flounder to better contribute to future generations of the fishery through increased reproduction and recruitment into the next year class within the fishery. While migration has been thought to peak in November in response to dropping water temperatures, warming waters may be shifting the migration later in the year. The initial November season closure in Texas was extended through December 15^th to account for these shifts. However, as warm waters continue to drive later migrations, the effectiveness of this approach may diminish.

To ensure the closure remains an efficient method to protect migrating Southern Flounder, a study was conducted by the Center for Sportfish Science and Conservation at the Harte Research Institute and funded in part by Texas Sea Grant. The study intended to inform fisheries management on current timing and trends of the Texas Southern Flounder spawning migration. The study was conducted in Aransas Pass and Packery Channel tidal inlets through which migration offshore is known to occur. Transect surveys were conducted weekly from September through December of 2022 and 2023. Weekly Southern Flounder densities were calculated to determine when movement through the inlets was greatest.

The study revealed most legal-size Southern Flounder migrated within the season closure’s protection, confirming the closure’s value as a management tool. Southern Flounder migrating before the closure were typically small females and males protected from harvest by the minimum size limit. However, 2022’s earlier cold fronts led to migration peaking in late October and November instead of early November and December as observed in 2023 when water temperatures dropped later in the year. This finding further indicates that warmer waters shift migration outside the closure and highlights the importance of regularly monitoring migration timing. Additionally, mature Southern Flounder that had not yet spawned were collected in January 2024, suggesting an even later migration time for some fish and, as a result, a later spawning period. Delayed spawning causes delayed return of larval Southern Flounder to estuaries. Fluctuating water temperatures impact larval survival, juvenile recruitment, and timing of the metamorphosis of the right eye to the left side of the body that is characteristic of flatfish survival strategies. Changes in these key life history events have the potential to negatively impact Southern Flounder survival before the fish even enter the fishery.

Another notable finding was a shift in the Southern Flounder sex ratio. Historically, a 7:1 ratio favoring females was observed in the study area, but this study observed a ratio of 4.8:1. This finding suggests potential masculinization of Southern Flounder populations which may lead to less successful spawning. Water temperature changes may be responsible, as water just 2°C above or below the optimum temperature during the juvenile stage causes genetically female Southern Flounder to develop male traits and reproductive anatomy, making females become males. Furthermore, the Southern Flounder fishery is dominated by females since males seldom reach harvestable sizes. This means all harvest consists primarily of females, which may also play a role in skewing the sex ratio. Sex ratio shifts may damage recruitment success and the overall population, further illustrating how climate change compounding with fishing pressures may be impacting Southern Flounder populations.

From spawning time to sex ratios, the climate-dependent life history of Southern Flounder illustrates the importance of adaptive management to fishery sustainability. Effective fishing regulations may be the difference between a healthy fishery and the continued decline of an economically and ecologically valuable species. Updated regulations, combined with consistent monitoring of life history and broader climate impacts, will ensure Southern Flounder continue to be enjoyed by Texans for many generations to come.